Pro Forma Financial Information (re: 2018 and 2017 acquisitions) (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended												12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Business Acquisition, Pro Forma Information [Abstract]
Net Income, As Reported	$ 8,509	$ 29,242	$ 29,180	$ 26,208	$ 26,573	$ 24,184	$ 23,434	$ 26,427	$ 24,629	$ 24,067	$ 22,992	$ 22,159	$ 93,139	$ 100,618	$ 93,847
Basic EPS, As Reported (in dollars per share)	$ 0.27	$ 0.93	$ 0.93	$ 0.83	$ 0.84	$ 0.77	$ 0.74	$ 0.83	$ 0.76	$ 0.74	$ 0.70	$ 0.67	$ 2.97	$ 3.19	$ 2.88
Diluted EPS, As Reported (in dollars per share)	$ 0.27	$ 0.92	$ 0.92	$ 0.83	$ 0.84	$ 0.76	$ 0.74	$ 0.83	$ 0.76	$ 0.74	$ 0.70	$ 0.67	$ 2.95	$ 3.17	$ 2.86
Increase (decrease) in reported revenue													$ 133,900
Increase (decrease) in reported revenue, percentage													36.80%
Operating revenue	$ 136,327	$ 128,643	$ 124,626	$ 108,230	$ 104,681	$ 92,800	$ 87,387	$ 79,103	$ 80,046	$ 74,608	$ 72,574	$ 71,066	$ 497,826	$ 363,971	$ 298,294
Effect of Exchange Rate on Revenue													(6,900)	2,100
Parent, Curepet, Healthcare Magic, Vertex, Oakstone, I3, Qatarlyst
Business Acquisition, Pro Forma Information [Abstract]
Revenue, Pro forma													584,105	605,649
Net Income, Pro Forma													$ 97,935	$ 122,269
Basic EPS, Pro Forma (in dollars per share)													$ 3.12	$ 3.88
Diluted EPS, Pro Forma (in dollars per share)													$ 3.11	$ 3.85
Increase (decrease) in unaudited pro forma revenue													$ (21,500)
Increase (decrease) in unaudited pro forma revenue, percentage													(3.60%)